There has been proposed an electrophotographic image-forming apparatus for use with a developer material mainly made of toner. Typically, the image forming apparatus has an electrostatic latent image bearing member or photosensitive member and a developing roller spacedly opposed to the photosensitive member. The developing roller has a cylindrical peripheral surface for supporting electrically charged toner particles thereon. In an image forming operation, an electrostatic latent image is formed on a peripheral surface of the photosensitive member. The electrostatic latent image includes an image portion which will be visualized and a non-image portion which will not be visualized. The charged toner particles are supplied onto the image portion of the electrostatic latent image due to a voltage difference between the image portion of the electrostatic latent image and the developing roller to visualize the image portion into a toner powder image. The toner powder image is transferred and then fused on a medium such as paper to result in an image product.
JP 05-11582 A discloses another image forming apparatus for use with a single component developer material in which an alternating voltage is applied to the developing roller so as to improve the movability of the toner particles from the developer roller to the photosensitive member.
In the meantime, the photosensitive member and/or the developing roller incorporated in the image forming apparatus can be eccentrically supported. This causes a variation of the gap between the photosensitive member and the developing roller during rotations thereof and thereby a variation of a magnitude of the electric field formed between the photosensitive member and the developing roller. As a result, a developing force which overcomes a adhering force of the toner particles onto the developing roller to jump the toner particles away from the developing roller can vary periodically, causing an unwanted density unevenness in the resultant image. The density unevenness may be reduced to a certain extent by a precise positioning the photosensitive member and the opposing developing roller, which in turn results in a significant cost increase in manufacturing and therefore is impractical.
The inventors of the present application have studied the generation of the density unevenness through experiments in detail. This showed a tendency that the density unevenness appeared more in dot images at a reduced alternating voltage and more in solid images at an elevated alternating voltage.
The reasons behind the fact are considered to be as follows. When compared the solid and dot images, the solid electrostatic latent image has a greater electric field than the dot electrostatic latent image. Therefore, the toner particles on the developing roller are attracted onto the solid electrostatic latent image than the dot electrostatic latent image, so that the dot image tends to suffer from more density unevenness due to the eccentricity of the developing roller under the reduced alternating voltage. Under the elevated alternating voltage, a sufficient amount of toner particles needed for visualization is attracted to both solid and dot electrostatic latent image. However, a part of the toner particles on the solid electrostatic latent image may be deprived therefrom by the enhanced electric field which electrically forces the charged toner particles from the photosensitive member back to the developing roller. Contrarily, the toner particles on the dot electrostatic latent image are maintained on the photosensitive member by an edge effect derived from an electric field generated at the edge portion of the dot electrostatic latent image, so that no visible density unevenness would occur on the resultant dot image.
As described above, the mechanism causing the density unevenness in the solid image differs from that in the dot image. Then, the voltage setting for preventing the density unevenness in the solid image differs from that in the dot image. Therefore, it has been considered to be rather difficult to prevent the density unevenness in both solid and dot images simultaneously.